International Journal for Quality Research 10(3) 523–546 ISSN 1800-6450

Cristiano Fragassa 1 Martin Ippoliti TECHNOLOGY ASSESSMENT OF MOULD CLEANING SYSTEMS AND QUALITY FINISHING Article info: Received 06.12.2015 Abstract: A modern tire merges up to 300 different chemical Accepted 07.03.2016 elements, both organic and inorganic, natural and synthetic. UDC – 332.05 During manufacturing, various processes are present such as DOI – 10.18421/IJQR10.03-06 mixing, calendering and extrusion, forming dozens of individual parts. Then, moulding and inside special moulds provides the tire its final shape. Since the surface quality of moulds strongly affects the quality of tire, mould cleaning is a fundamental aspect of the whole tire production and cleaning techniques are in continuous development. This investigation proposes a global technology assessment of tire mould cleaning systems including uncommon solutions as multi-axis robots for cleaning on board by laser or dry ice, or ultrasonic cleaning which use cavitation. A specific attention is also focused on the industry adoption of spring-vents in moulds and how they are influencing the development the quality of final products. Keywords: Tire Industry, Moulds, Laser cleaning, Dry ice cleaning, Ultrasonic cleaning, Spring-vents

1. Overview of tire 1 patented its in 1946 and started the sales since the end of the 40s. Radial 1.1. Brief history were used in the US only from the 60s (Eberhart, 1965). The first pneumatic tire was patented by Robert William Thomson in 1845. He also 1.2. Tire types applied for a patent in the US, which he received in 1847 (U.S. Patent No. 5,104, The tires can be classified according to their 1847). This tire, with an inner tube, failed to structure. It is then possible to distinguish 4 replace the solid rubber tire due to resistance types: problems. John Boyd Dunlop, developed a  Pneumatic tire: inflated hollow tire, similar but improved pneumatic tire in 1888 retains its shape due to the air pressure, (British Patent No. 10,607, 1888). The radial and equipped with air chamber or tire was patented in 1915 by Arthur W. tubeless (CBP, 2014). Savage (U.S. Patent No. 1,203,910, 1915).  Semi-pneumatic tire: hollow tire which After the expiry of Savage’s patent, is not pressurized. Lightweight, puncture-proof and shock-absorbing,

1 this tire is commonly used, for example, Corresponding author: Cristiano Fragassa email: [email protected] on lawn mowers, shopping carts and wheelbarrows (CBP, 2014).

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 Solid tire: Made by molding, a solid tire  To provide traction; is composed of several layers of  To ensure stability and directional different thickness and compound, control of the vehicle. which form a composite structure During several decades other characteristics (Figure 1). Steel rings are often used as have been added to these basic functions, on reinforcement (Suripa and demand of the market, such as increased Chaikittiratana, 2008). Solid tires are wear resistance, homogeneous response to among the most important engineering each condition and increasing security. components in heavy industry (Suripa Finally, the tire was expected to contribute to and Chaikittiratana, 2008). They are reducing fuel consumption. As emerged mainly used in forklifts and in various from the research prepared by the Centro Pio types of industrial vehicles, but also on Manzù (1987), this led to a change of karts, scooters and lawn mowers (CBP, perspective, particularly in the automotive 2014). industry. The tire cannot be considered just a mere commodity anymore, but it assumes the role of a real customer technical support. In the automotive sector, the quality of a product does not generally depend only on its performance, but it is a merging of different factors between technological aspects and clients satisfaction (Fragassa et al., 2014). Tires are not an exception, there are no compromises as regards their safety and reliability (Peled, 2015), and they are subject to constant evolution.

Figure 1. Cutaway of a solid tire [Courtesy of Continental]

Cushion tire: It is a solid tire, however equipped with a sealed internal air space Figure 2. Schematic structure of a radial tire rather than containing pressurized air in [Courtesy of Wikimedia Commons] order to maintain the shape (CBP, 2014). The structure of a tire may vary depending 1.3. Structure of a pneumatic tire on the manufacturer and the intended use. It is possible to identify some basic elements The main functions of a tire are (Gough, (Figure 2): 1981; Peled, 2015):  Tread: Part of tire in contact with the  To support the weight of the ground. Its surface is suitably shaped vehicle; and can be engraved to improve road  To absorb the irregularities of grip and water expulsion (Koštial et al., grounds; 2012).

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 Steel belts: Reinforcements made of resistance and better high-speed steel wires coated with rubber. They are performance. By contrast, the greater positioned between the carcass and the complexity of construction leads to an tread. Steel belts, crossed with each increase in costs (Lindenmuth, 2006). other, optimizes the directional stability Bias belted tires are bias tires with belts and (Koštial et al., added in the tread region (Lindenmuth, 2012). 2006).  Nylon cap: One or two nylon belts  Shoulder: Part between the sidewall and placed over the steel belts and inclined the tread. Help maintain a smooth belt at 0° to the rolling direction. They are contour and insulate the body plies from used in high-end tires and should the belt edges (Lindenmuth, 2006). improve the resistance to the speed.  Sidewall: It bears most of the weight of  Carcass: It bears the inflation pressure. the vehicle and, for this reason, it must By wringing, it transmits forces between be strengthened properly. It also protects the tire and the ground. It consists of the carcass from the weather conditions several crossed plies (Shuy, 1964). Plies (Koštial et al., 2012). are rubber layers with a low elastic  Bead chafer: Side part in contact with modulus, which are reinforced by high the ’s . It partially envelops the modulus cords. These cables can be plies Figure 2 and is mainly made of natural textile, synthetic polymers, metal. It should ensure the adhesion especially Nylon, polyester, Rayon, between tire and wheel and improve air glass fiber or be steel cables (Koštial et tightness. al., 2012; Lindenmuth, 2006; Gough,  Bead wire: Ring consisting of bronze- 1981). The number of plies depends on coated or brazed steel wires coated with the type and the size of the tire and by rubber (Koštial et al., 2012; the inflation pressure (Gough, 1981). Lindenmuth, 2006; Shuy, 1964). It Plies are turned around bead (Figure 3). should ensure the between tire The height to which the plies are taken and rim and transmit power. after they have passed round the bead  Bead : Also called bead apex, its can be low or high, or the plies' characteristics affect handling extremity can extend across together, as (Lindenmuth, 2006). in Figure 4. In this case the plies can be  Liner: Inner layer that functions as an arranged to give extra carcass strength inner tube. in the central region of the tire compared with the sidewalls (Gough, 1981). Standard types of tires are diagonal (or “bias”), radial and bias belted (Figure 5). In diagonal or bias tires plies are crossed at 36° (Eberhart, 1965). In radial tires, as suggested by their name, the plies should be positioned radially, thus with an angle of 90° with respect to the rolling direction. As shown in Gough, 1981, this solution will wear poorly because Figure 3. Detail of a typical bead (Gough, the shear modulus of cord-rubber is very 1981) small. The optimum angle is between 70° and 75°. Radial ply tires generate less heat during use, have lower rolling

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Figure 4. Different plies’ fixing arrangement (V. Gough 1981)

Figure 5. Type of tire-casing (Koštial et al, 2012)

1.4. Composition of the compound depends on the intended use of the tire. For example, for a competition the objective The compound is composed of several may be to optimize performance. For long elements (Table 1), a typical car tire contains distance vehicle, as truck and bus, the usual about 60 raw materials (Lindenmuth, 2006). aim is to minimize the rolling friction, then The compounds can vary depending on the to limit fuel consumption and tire usury. manufacturer and the objective sought, that

Table 1. Compound composition (Evans, 2006) Ingredients Car Lorry OTR Rubber/Elastomers 47% 45% 47% Carbon Black 21.5% 22% 22% Metal 16.5% 25% 12% Textile 5.5% -- 12% Zinc Oxide 1% 2% 2% Sulphur 1% 1% 1% Addictives 7.5% 5% 6% Carbon-based materials, total 74% 67% 76%

Elastomers: These are materials able to (polyisoprene) is an resume the original shape and size, even if elastomer resulting from the coagulation of greatly deformed by stress, once such stress the latex, an emulsion of aspect sticky has been removed (Campbell, 1963; Mark, proceeds from some tropical plants, typically 1943). They are characterized by a mainly Hevea brasiliensis (Niyogi, 2007a). In the amorphous structure, with low elastic processing of the latex various chemicals are modulus. They can withstand large employed, such as ammonia as a deformations before arriving at break. preservative and formic acid as a coagulant

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(Rodgers, 2015; Drury, 2013). Rubber is  High static tensile strength (15-22 composed of long, string-shaped chains that MPa); arrange themselves in carbon-hydrogen  High elongation (600-900%); bonds (Raja, 1990). Natural rubber has good  Excellent elasticity at low abrasion resistance, it is insoluble in many temperature; solvents and has good hysteretic properties,  Poor ozone and degradation but is little resistant to fatigue (Rodgers, stability; 2015).  Good crafting because of excellent A compound base of natural rubber (Table crude adhesion. 2.) has the following properties: (Renner and Pék, 2011):

Table 2. Example to the composition of a natural rubber base compound (Renner & Pék, 2011) Ingredient Mass rate Natural rubber 75.1% Wood-tar 1.5% Resin oil 0.8% Stearine acid 1.1% Zinc-oxide 18.8% Sulphur 2.3% Other accelerators 0.5%

Table 3. Rubber % by Weight in New Radial Passenger (Evans, 2006) Application/Component %

Tread 32.6% Base 1.7% Sidewall 21.9% Bead Apex 5% Bead Insulation 1.2% Fabric/Fibre Insulation 11.8% Steel Cord Insulation 9.5% Liner 12.4% Undercushion 3.9% Total 100%

Table 4. SBR properties (S. Raja, 1990) Excellent Good Poor Abrasion resistance Tensile/elongation Ozone resistance Water resistance Flex resistance Oil resistance Cost Tear strength Solvent resistance Compression set Resilience Fuel resistance Low temperature High temperature (120 °C and above)

Synthetic rubber derive from the compounds with physical, mechanical and polymerization of hydrocarbons, such as chemical properties that are very different isoprene, butadiene and chloroprene between them. The monomers may be used (Gilliland, 1944). These and other monomers pure or with the controlled addition of may be mixed in different parts, obtaining impurities or additives, so as to obtain the

527 desired characteristics. , surface/volume ratio and contains a low initially also known as "cold rubber", is percentage of PAH (polycyclic aromatic better than natural rubber as far as hydrocarbons). It is used both as a pigment is concerned (Society for Science & the and as a filler for reinforcement. It Public, 1950). Another characteristic of the contributes to heat dissipation, thus elastomers is the production of heat under increasing the life of the tire. As example, stress. For modest elongations (about 200- the effects of the addition of a 30% of carbon 300%) the work required for extension black to a SBR are reported in Figure 6. coincides almost completely with the Carbon black is divisible into series, entropy decrease and has as a consequence depending on the size of the particles. In the the production of heat. This production tire industry, the most used particles are results from the random orientation of the comprised between 0.01 and 0.2 microns. chains of the elastomer (Campbell, 1963). (Clarke, 2014a). The percentage of carbon Among the many synthetic kinds of rubber, black in the tire compound varies depending it is certainly cited the SBR. Considered the on the manufacturer and type of tire. greatest discovery in the field of elastomers, According to Downs et al., 1996, this it is the most important rubber general percentage varies from 16 to 36%. A more purpose as well as the most produced recent study, Evans (2006), shows a worldwide (Niyogi, 2007a; Raja, 1990). In percentage of about 22%, but also including 1990, 40% of synthetic rubber used silica. Due to its high density, carbon black industrially was SBR, and the tire industry may constitute more than 50% of the weight consumes most of the production of SBR. of compound (Campbell, 1963). The rubber (Niyogi, 2007a; Raja, 1990). The industry accounts for 90-95% of world International Institute of Synthetic Rubber production of carbon black. Of these, 80% is (HSR) classifies SBR in six different series. used in the production of tires and related Among its properties (Table 4), those which product (Niyogi, 2007b). have most influenced success are the low cost, high abrasion resistance, which makes it ideal for passenger car tires, and a level of uniformity of the finished product comparable with that of natural rubber (Niyogi, 2007a). There are four main types of elastomer which are used in tires: Natural rubber, the already mentioned stirene- butadiene rubber (SBR), polybutadiene rubber (BR) and (together with the halogenated butyl rubber). First three are used for the external parts of the tire, such as hip, shoulder and tread. Butyl rubber, impermeable against gases and air (Raja, Figure 6. Vulcanized SBR with 30% carbon 1990), is used in the liner. SBR is also used black compared with natural SBR in liner's compound, because of its resistance (Campbell, 1963) to water and compression (Raja, 1990). In a modern car’s tire 15 or more rubber Silica: In the form of fumed silica. If used as compounds are used (Lindenmuth, 2006). a filler increases viscosity. Furthermore, compared to carbon black, it guarantees a Carbon black: Material obtained from the lower rolling friction (Clarke, 2014a). This incomplete combustion of heavy oil, such as ensures lower consumption and better-wet tar or fossil tar obtained from ethylene grip. In the past, this material was suffering cracking (Clarke, 2014a). It has a high from a worst abrasion resistance compared

528 C. Fragassa, M. Ippoliti to carbon black, but this gap was are added, such as malleability, flexibility, subsequently filled. The use of silica reduces density, tensile strength or adhesiveness. fuel consumption by up to 2.6% and the Primarily are oils, softeners, or peptizers overall environmental impact by 11%. (Bebb, 1976). Called process oils in the According to Crump (2000), the silica has rubber industry, they improve the replaced 25% of the carbon black in tire mechanical performance of tires and compound. properties at low temperatures. compose up to 30% of the compound. Antiozonants: They are meant to stop or slow down the process of deterioration and Vulcanizing agents: During the cracking caused by exposure to ozone, an vulcanization process, they transform rubber allotropic form of oxygen, that is contented from elastic into material. Sulphur is in the atmosphere (Ozone damage testing, the most common agent and is used for 2010). The ozone quickly causes the break natural and many synthetic types of rubber of the unsaturated double bonds of rubber (Niyogi, 2007b). In the case of some specific components (Niyogi, 2007b), such as elastomers, metal oxides, difunctional aldehydes and ketones. This causes a compounds or peroxides are mixed as agents reduction of the polymerization, which can instead of sulphur (Niyogi, 2007b). lead to the formation of true cracks on the surface. A negative consequence is the Accelerators: They accelerate the rusting of metal components of the plies, in vulcanization process. In the absence of addition to the damage to the tire, due to them, it would take 5 hours at 140 °C to humidity. Actually, the most effective complete the process (Niyogi, 2007b). The antiozonant is para-phenylenediamines principal accelerators used in the (Niyogi, 2007b). vulcanization with sulphur are: Aldehyde – Antioxidants: Waxy materials. They amines (also used as secondary ), prevent oxidation, and thus the degradation Guanidines, Benzothiazoles, Sulfenamides, of the tire caused by exposure to oxygen and Dithocarbamates, Thiurams, Xanthates and ultraviolet rays. Oxygen acts more slowly Morpholines (Niyogi, 2007b). than ozone, and carries a softening of natural Activators: They accelerate the vulcanization rubber or a hardening in the case of SBR, process by activating accelerators. The and more generally to the progressive loss of activators can be divided into inorganic and physical properties. The process in this case organic. Inorganic activators are: Zinc oxide, can also from the air chamber, and from the hydrated lime, litharge, red lead, white lead, liner spread outwards. Natural rubber magnesium oxide. The most common is zinc contains natural antioxidants, while the oxide. This is generally combined with a synthetic rubber lacks of them. The use of fatty acid, to form a soap-like compound in synthetic rubber requires the addition of the rubber matrix. Organic are normally used antioxidants (Bebb, 1976). Synthetic rubber in combination with inorganic. They are such as butadiene-styrene and isoprene are generally monobasic fatty acids or mixtures very sensitive to oxidation. On the contrary, of the types, stearic, oleic, lauric, palmitic butyl rubber and ethylene-propylene-based and myristic acids, and hydrogeneated oils rubbers in particular, are much less sensitive from palm, castor, fish, and linseed oils (Bebb, 1976). Principal antioxidants are (Niyogi, 2007b). amines, such as products of the reaction of Retarders: They prevent scorch during diphenylamine, and phenolics (Niyogi, processing and prevulcanization during 2007b). storage. They work by lowering the pH of Plasticizers: They are elements that modify the compound and do not work at some properties of the compound to which vulcanization temperature. Generally they

529 are organic acids such as salicylic acid and silos, are inserted in a closed Banbury mixer phthalic anhydrid (Niyogi, 2007b). (Shuy, 1964). The major components are dosed via a computer-controlled systems. 1.5. Reinforcement materials Minor ingredients are suitably dosed. During the second phase other materials such as As already mentioned, in tires and accelerators and curatives are added. In the particularly in the carcass various reinforcing mixer the blend is worked from the rollers at materials are used, especially textile fibers temperatures above 100 °C. The process is and metal wires: called mastication. Mastication, which is fundamental, increases the plasticity of the Nylon: Mainly type 6 and 6,6. Nylon is rubber and decreases its viscosity (Bebb, strong and resistant to heat and moisture. It 1976). Both hot mastication (above 130 °C) suffers from progressive elongation and and cold mastication (below 100 °C) are flatspotting (Lindenmuth, 2006). more efficient than warm mastication (ca. Polyester: Commonly Poly (ethylene 115 °C) (Bebb, 1976). Other studies move succinate) (Koštial et al., 2012). Its down the values of high, low and warm characteristics are: high strength with low temperature, respectively, at 120-140 °C, 30- shrinkage, low service growth, low heat set 50 °C and 100 °C, but they agree on the and low cost. Compared to Nylon and greater efficiency of mastication at high and Rayon, it is less resistant to heat low temperatures (Niyogi, 2007a). At high (Lindenmuth, 2006). temperatures, the break of the chemical Rayon: It is used primarily by European tire bonds is caused by oxidation. In particular, manufacturers and in some run-flat tires. It over 100 °C, some plasticizers used in the has good dimensional stability, heat industry become oxidation catalysts. At low resistance and it is easy to handle. By temperatures, however, the formation of free contrast, in addition to being expensive, it is radicals during mastication can cause this sensitive to moisture (Lindenmuth, 2006). breakage. (Niyogi, 2007a; Bebb, 1976). Aramid: It is 2 to 3 times stronger than Then the mixer is opened and drops the polyester and nylon. It can be used as a light compound in an open mixer, consisting of weight alternative to steel cord. Extremely two horizontal cylinders, which improves strong and stiff, it is heat resistant. As well mixing and diffusion (Figure 8). By as expensive, it is difficult to cut calendering the compound is converted into (Lindenmuth, 2006). rubber sheets (Lindenmuth, 2006). The produced sheet is cooled by passing it Steel cords: Carbon steel wire coated with through a water tank. This is the rubber part brass. The cables are twisted in of the tire, and will form elements such as multifilament strings. Strong and stiff, they tread, the sidewall and the plies. This sheet require special manufacturing processes and will undergo a first grinding process. During are sensitive to moisture (Lindenmuth, processing of the compound are prepared 2006). reinforcements of plies. These are mainly

textile fibers, especially polyester but also 2. Tire construction nylon or rayon. Such fibers undergo special treatments to enhance rubber adhesion. All 2.1. Processing raw materials the moisture is also removed from the fibers. Perfect adhesion is fundamental to achieve The processing of the tire components is satisfactory performance. This adhesion is divided into two phases (Figure 7). In the obtained by calendering (Shuy, 1964). The first stage, raw materials such as plasticizers, fibers are maintained at a predetermined carbon black and silica, normally held in voltage and are continuously pressed by two

530 C. Fragassa, M. Ippoliti steel rollers. The compound is added through clad in brass to promote the adhesion of the the opening between the two rollers, both rubber. The density of the warp varies above and below the fibers. The mix of the depending on the type of tire but must be compound and the resulting fibers passes constant. The obtained metallic fabric is through other rollers up to obtain the mutual combined with the compound again by penetration and the desired thickness. During calendering (Figure 9). Nylon belts are the process, parallel and spaced apart cotton obtained by a single broad band of nylon, or yarns are also added to the plies. The with a wide waistband at most 8 mm, which function of these strands is to highlight the during assembly is wound in a spiral around direction of the fibers. In addition, the cotton the tire. The bead wire is obtained by coiling absorbs much moisture instead of plies, the steel wires. The wires are copper-coated protecting them. The bead filler and the tread or brazed to enhance rubber adhesion are produced by extrusion. The tread is (Koštial et al., 2012). The number of wires devoid of notches, which will be etched in and of turns of winding depends on the size the moulding step. The steel belts are of the tire. Once impregnated with rubber, intertwined within the plant. The individual the spiral is firm. Welding is not necessary. cables, formed from steel wires braided, are

Figure 7. Schematic representation of the production process of a tire [Courtesy of WBCSD]

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Figure 8. The open mixer, in which the compound, which came out from the Banbury mixer, is inserted [Courtesy of HSE]

Figure 9. Calendering of plies [Courtesy of HSE]

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2.2. Assembly and vulcanization absent in natural and synthetic rubbers, which are thermoplastic in nature. During The various semi-finished products flow into vulcanization, normally hot performed, the assembly zone. A packaging machine, sulphur binds with various tire chains, that may be automatic or manual (Peled, creating cross-links. Vulcanization of rubber 2015), is composed of two rollers. On the can be accomplished by different moulding first roller liner, carcass, wires and sidewalls techniques but according to Raja, 1990, the are assembled. On the second roller belts and three most common are compression, tread are assembled. The two parts are transfer, and injection moulding. The assembled together and form the green tire vulcanization of the tire takes place in heated (Shuy, 1964). The components are pressed flat presses. Before insertion of the tire, the together from the machine. An automated press is heated by steam at 180-200 °C. The machine can assemble a tire in less than a press contains an air chamber. When the minute. The tire is ready for the inspection press is closed, the air chamber is inflated may be subjected to curing. The and presses the tire against the mould vulcanization is a process in which the (Figure 11). The pressure can reach 22 bars. natural rubber binds with the sulphur. Ruber The moulding of a car tire lasts about 10 turns from a plastic into an elastic material minutes that of a tire for transport read lasts (Clarke, 2014b). Also, as reported in about twice. The tire is then extracted for the Campbell (1963), the extension work, and inspection. The extraction of the tire from consequently the production of heat under the mould is one of the most delicate stress, is intensified (Figure 10). More moments of the process (Gavine, 2011). Tire generally, with the vulcanization is meant mould release agent are used to facilitate the any chemical reaction that produces the operation, normally sprayed on the green same effect on synthetic rubber. This tire, on the mould or in the form of mould transformation occurs with the formation of coating. The curing lasts from 1.5 to 16 cross-links (Clarke, 2014b), rare in nature or hours (Degraeve et al., 1997).

Figure 10. Effect of vulcanization of a synthetic elastomer with no fillers (J. Campbell, 1963)

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vulcanization process are filled with the uncured rubber. Most of them remain attached to the tire once this has been extracted, going to form the classic “hairs” on the surface of the tires (Figure 13). This “hairs” are called “vents”, too (Lindenmuth, 2006; Shuy, 1964). Other vents instead remain inside the mould, going to occlude the ventilation channel. This causes various problems. First of all, the cleaning of such channels is required. If all the channels of a gusset were closed, the air inside would Figure 11. Tire molding [Courtesy of remain inside the mould and form a bubble. Bridgestone] This bubble prevents proper tread. The tire would be scrapped, with economic 2.3. Tire mould consequences due not only to the loss of the tire but also to the problems of disposal of Tire moulds are very complex and must be the same (Young, 2000). Consequently, the made with extremely tight tolerances. A vent should be cleaned often (Kerkstra, mould is composed of several parts (Figure 2016). The manual cleaning of such channels 12). Two elements called sidewall give requires a high level of labor. Each mould is shape to the tire sidewalls. The sidewall have a unique element, the cleaning process the form of centrally perforated discs (a cannot be automated. Each channel is to be “donut” shape, according to Chih-Hsing et inspected and possibly punctured with hand al. (2013) and are made of steel. A number drills. The drill bit may break inside the of elements, called segments, shape the tread channel (Young, 2000). In this case, the and are made of aluminum alloy. Segments’ mould must be repaired. For cleaning grooves can be directly carved out by multi- equipment are used high quality bits which axis CNC machining or the grooved combine high mechanical performance, such segments can be made by precision casting as elasticity and resistance to high (Chih-Hsing et al., 2013; Lecuiller, 2010). temperatures, with very specific dimensions The grooving is a highly complex machining both in length and in diameter. Bits have process and normally requires 5-axis CNC usually diameters between 0.4 and 2 mm, machining (Chih-Hsing et al., 2013). The with a cutting length of the propeller 20 to price of a mould can be up to $ 60,000. It 100 mm. Tips are usually made of high- must be realized a mould for each type of speed cobalt steel, or are equipped with tread and for each tire size. Due to economic coatings that increase the elasticity. These reasons it is not possible to get a press for tools are very specific, far from the standard each mould. The change of the mould is a production. In case of breakage of the tip, it long operation. During the vulcanization the may be necessary to remove the part of the tire inflates inside the mould, adhering to it mould containing the channel and its in all its parts. The air inside should then exit replacement with a perforated insert. The from the mould. Each plug is equipped with vents that remain attached cause aesthetic 2-3 channels, for a total of 1,500-2,000 problems. Manufacturers are forced to cut channels, called “vents”, in a single mould. away these vent to stick labels. For this They are divided into “vents” (1-2 mm) and operation is used a machine called vent “microvents” (0.5-1 mm) (Young, 2000). trimmer (Shuy, 1964). These channels behave as small extrusion dies (Young, 2000). During the

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This method is expensive and increases rubber waste. Trimming a single tire produces 40 g of rubber. Therefore, in a plant that produces 10 million tires a year, tire trimming produces 400 metric tons of extra waste (Spring vent technology, 2012). After 2010, the simple ventilation channels have been replaced by spring-vent (Figure 14). The spring-vent are little clips with the form of a screw, in number of 2-3 to gusset, which protude from the surface of the mould allowing the air outlet, but at full inflation Figure 12. Tire mould [Courtesy of range in coupling and prevent the compound Tianyang Mold Co., Ltd] from entering inside the duct. The spring- vent can be mounted both on new and on old moulds, both on steel and on aluminum moulds, with a price increase of 5-10% on the total cost of the mould (Spring vent technology, 2012).

2.4. Release agents

To facilitate the extraction of the tire, improve the quality of the tire and increase the wear resistance of the mould are used release agent, in form of coating or mixed together with the tire compound (Okada et al., 2014). Single use release agents: Release agent are sprayed on mould. Principally are non- reactive silicones, soap solutions/suspensions and oils (Wolers). A certain amount of release agent remains Figure 13. Tire vents [Courtesy of Doug attached to the tire, causing various Chase] problems.

Generally, the problems of this type of agent are the following (Wolers):  They require an extra step in the production process;  They can interfere with the compound flow in the mould, which may lead to poor knitting;  Depending on the type of material being used, they can cause mould

Figure 14. Spring-vent [Courtesy of fouling; Gottschol Alcuilux CZ Ltd.]  They may interfere with the product surface;

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 They contain VOCs that may be 2.5. Tire markings dispersed in the work environment and cause health problems. In the post-production phase, lines and Semi-permanent release systems: These colored points, that serve different functions, coatings are designed to release more tires are plotted on the tires. On the surface of the with a single application. Technically the tread of a new tire colored stripes and coating, in the form of film, reduces the alphanumeric codes are visible. Such signs adhesion of the rubber to the mould and the are printed on the tread when it is still in release agents do not remain on the surface strip form, and used to identify the exact of the tire, enabling to obtain a better measure and model of tire to which such a moulding quality. Chemically, the film tread is intended (Figure 15). On the adheres to the mold due to Van der Waals’ sidewall of the tire, a red dot or a red and a forces (Wolers). The tread pattern and yellow dots are marked (Figure 16). markings are better defined. This is Together with a white dot marked on the important because the aesthetics of the tire is rim, it is used in the match mounting. In increasingly important in the competitive tire Mavrigian (2008), it is explained that match market (Gavine, 2011). The latest generation mounting use to position the tire on the of semi-permanent release agent has wheel in such a way that minimizes or additional features, such as increasing the deletes the final combination of radial force number of tires that can be moulded before it variation and/or imbalance. A red dot shows is necessary to clean the mold. This reduces the tire radial, runout high point, while a downtime. This result is obtained because yellow dot indicates the tire point of least the coating acts as a barrier, slowing the weight, from a balance standpoint formation of fouling. (Gavine, 2011). Older (Mavrigian, 2008). An employee usually types of semi-permament release agents are performs the identification and quality solvent-based, this causes problems of control of the dots, but for the same task are environmental impact and problems of being studied automated systems that use health and safety at work. In some countries artificial vision (Dias et al., 2014). products containing these solvents have been banned. Solvents have been therefore replaced by other materials. Initially they were highly flammable products, that obviously caused new problems. However, safe products, with the same performance, were finally developed (Wolers). Permanent release agents: Commonly polytetrafluoroethylene, known by the trade name Teflon. The first permanent Teflon coatings appeared in the early 2000s (Young, 2002). The permanent coatings perform the same functions of semi-permament, mainly facilitate the extraction of the tire and slow down the formation of fouling. Chrome (Cr), chromium nitride (CrN) and titanium nitride (TiN) are used for permanent and semi-permanent coatings (Okada et al., 2014). Figure 15. New tire with colored stripes for tread's identification

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 To avoid the formation of surface irregularities during the drying phase due to the problems previously listed;  To improve durability and reliability of the mould. Generally, as the tire arrives at the moulding step, the majority of the cost associated with its production has already been spent. A tire that is damaged in this phase, for example 3. Tire moulds cleaning because of a dirty mould, must therefore be discarded: This obviously has a high 3.1. Generalities financial impact as a consequence (Young, 2000). For this reason, proper mould A tire mould is cleaned every 1,000-3,000 cleaning is fundamental. With technological tires moulded (Schreiber, 2011). This progress, the amount of inorganic materials number is the maximum limit to maintain an used in tires has increased. These materials acceptable moulding quality. Above that may only be cleaned with acid products. limit, it is necessary to remove the mould This made obsolete the old alkaline washing fouling (Figure 17). They are mainly the machines, which were able to effectively result of the chemical reactions of sulphur clean only organic residues. The moulds do and zinc oxide (Young, 2000). Furthermore, not have a single layer of dirt, but thousands pieces of rubber often remain attached to the layers, one for each tire moulded. Each layer mould (Okada et al., 2014). is formed from an organic part and an The reasons why it is important to keep inorganic part. Alkaline products can then clean a mould are: remove only a certain number of layers of  To keep a proper surface finish of dirt. Beyond this number, the inorganic bead chaffer. This ensures proper compounds form a barrier. For the same adhesion of tire to wheel; reason, you cannot clean a mould only with acid products, it would be stopped by  To make tire code clear and organic dirt. Consequently, in the chemical readable as required by law cleaning are used alternately acid and (Young, 2002); alkaline products.  To keep the logo and the name of

the manufacturer of the tire clearly legible (Young, 2002);  To keep clean the vent or the spring-vent (Young, 2002);  To keep clean the contact surfaces between the sections of the mould, so there are no interruptions in tire surface and tread design (Young, 2002);  To ensure the coupling of the fields

at high pressures; Figure 17. Tire mould partially cleaned. The  To reduce the load on mechanical brown part is still dirty [Courtesy of Clean attacks of segments and other Lasersysteme GmbH] components (Young, 2002);

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3.2. Cleaning technologies significant difference between the two configurations (Young, 2002). Even with methods "slightly abrasive", after a certain number of cleanings, the mould is too ruined and must be discarded. There are also other problems caused by these systems. Residues of abrasive material may be trapped and modify the surface of the mould. This can prevent proper adhesion of the chemical release agents that must bind to the metal surface (Young, 2002). If release agents do Figure 18. Sandblasting of a sidewall not come in contact with metal but with [Courtesy of Eco Blast] other materials, they cannot penetrate into the mould. Their action runs out in a number of cycles lower than those expected.

Figure 20. Hand dry ice blasting [Courtesy of Melbourne Dry Ice Blasting] Figure 19. The two possible systems for blasting cleaning technologies (Foster, 2005) Dry ice blasting: This system consists of a

hand or automatic gun that shoots dry ice Sandblasting: Cleaning systems by against the mould (Figure 20). The dry ice is sandblasting are inexpensive, easy to install fired at a temperature of -78.5 °C against the and maintain (Young, 2002). In the case of mould at a temperature of 180/200 °C. The hand sandblasters, the operator should not temperature difference vaporize the dry ice have much experience. The sandblasting can before it touches the surface of the mould only be done in closed environments, to (Foster, 2005). The shock waves generated prevent the spread of fine dust in the by these explosions easily removed the workplace and to easily recover abrasive fouling. The dry ice blasting can also be used materials used in cleaning (Young, 2002). In on moulds at room temperature. Cleaning of the cleaning of tire moulds cleaning “hot” mould is 3-4 times faster than cleaning materials classified as “midly abrasive”, of “cold” mould (Young, 2002). In such as glass, plastic, metal and ceramic, are particular, below 65 °C, the fouling is no used (Young, 2002). The particles impacting longer a glass-like material, easy to crumble, against the mould, due to the kinetic energy but it becomes a very dense visco-elastic and the surface hardness, are fragmented. material that absorbs the impact energy of The fragments bouncing on the surface CO explosion (Young, 2002). The impact contribute to cleaning. The cleaning effect is 2 energy depends on the weight and the speed therefore not limited to an impact per of the particles. The dry ice used is of two particle (Young, 2002). A sandblaster system types: Flakes or pellets. Flakes as big as can be of two types: “Two-hose” or “single- sugar grains, are obtained by shaving with a hose” (Figure 19). Basically, there is no

538 C. Fragassa, M. Ippoliti machine with rotating blades a standard 60 using 5-axis robot. It may also be used off lb (27 kg) dry-ice block (Foster, 2005; board, both on high temperature moulds than Young, 2002). The flakes have a low density on room temperature mould. Furthermore, and do not guarantee high level of the laser does not generate secondary waste cleanliness (Foster, 2005). For maximum (Gavine, 2011) and do not have residual impact energy are used pellets of high powder on the surface. Compared to dry ice density, obtained by extrusion and cutting of blasting, it has less environmental impact, a string of dry ice. Pellets are available in with lower energy consumption, lower several sizes ranging from 1 mm to 3 mm in production of CO2 and minimum noise diameter (Foster, 2005). As for the pollution. The cost of a single cleaning sandblasting, there are two types of dry ice operation is about 3 euros, compared to 20 € blasting systems: “Two-hose” or “single- of dry ice blasting (Schreiber, 2011). The hose” (Figure 19). However, on the contrary investments required are high, 2-3 times of the sandblasting, there is a big difference higher than those required by an on-board between the two systems (Young, 2002). In robotic dry ice blasting, 25-30 times higher single-hose system, the exhaust Mach than a hand dry ice blasting. A problem of number of particles is about 1.0-3.0. While, cleaning laser is its difficulty to clean vent or in two-hose system the exhaust Mach very small cavity, in particular it is number is only slightly supersonic (Foster, problematic to clean tread with complex 2005). The single-hose systems, due to the designs (Young, 2002). Laser technology increased exhaust speed of the particles, are does not allow a high level of cleanliness in more efficient. Dry ice blasting is not the case of moulds with spring-vent that are capable of eroding the surface of the mould not cleaned by the laser beam. Indeed, the (Young, 2002). The automatic cleaning dry high temperature of the laser carbonize any ice blasting can be carried on board, and residue of oil and this blocks the spring-vent. without removing the mould from the press. In this way, they reduce downtime and avoids the formation of a flow of secondary waste (Foster, 2005; Young, 2002). A dry ice blasting can clean vent very small, up to about .03 inch, or 0.7 mm (Young, 2002). The extreme noise is among the critical points. In the case of hand dry ice blasting, there may be laws that protect the operator from loud noises (Young, 2002). The dry ice blasting can cause dusty conditions. Vacuum systems are required. This technology began to spread among all the large tire manufacturers from the mid-80s (Young, 2002). Laser cleaning: Cleaning technology that uses high power laser, up to 10 MW (Schreiber, 2011), to break down fouling. Figure 21. Automatic laser cleaning The affected area is equal to the size of the [Courtesy of 4JET Technologies GmbH] laser beam (Schreiber, 2011). The mould partially absorbs the laser. The substrate of Alkaline washing machine: In these the mould does not suffer mechanical stress, machines, the alkaline-based cleaning chemical stress or heat stress (Schreiber, product is sprayed against the mould to 30 2011). Laser cleaning can be used on board, atm.

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Figure 22. Operating principle of ultrasonic cavitation [Courtesy of Ultrawaves GmbH]

Ultrasonic cleaning: Technology that uses For the cleaning of moulds are employed ultrasonic sound waves whose frequency is frequencies comprised between 20 and 40 higher than the audible to the human ear. kHz. Ultrasound waves vibrate the fluid, Conventionally, ultrasounds are sound waves which compresses and expands alternately. with a frequency above 20 kHz. Ultrasonic During the compression phase, the positive cleaning technology is fairly old, it is used pressure makes them closer together the industrially since the 50s and has become molecules of the liquid, while in the economically viable since the 70s. It is used expansion phase the negative pressure does to wash small metal parts such as jewelry, move the molecules. When expanding is lenses, optical parts, watches, coins. It serves exceeded the tensile strength of the liquid, in mainly to wash away polishing pastes, which the liquid are formed of the cavity containing are a mixture of organic and inorganic steam. These cavities are called cavitation components. The ultrasonic cleaning is also bubbles. If theoretically this phenomenon is used on dental instruments, surgical formed with difficulty in pure liquids, in instruments, tools, weapons, electronic reality the impurities substantially facilitate components, circuit boards and moulds. the formation of bubbles. In the ultrasonic Specifically, ultrasound is used to create the cleaning are employed chemicals that phenomenon of cavitation. The cavitation is facilitate the formation of bubbles, the formation, growth and collapse of increasing the vapor pressure and at the same micron-sized bubbles, resulting in the release time decreasing the viscosity and the surface of enormous energy (Figure 22). The tension of the liquid (Adewuyi, 2001). The temperature can exceed 5000 °C and small size of the bubbles and short times of pressure of 700 N/cm2 (Johnson, 2007). cleaning ensure the cleaning of the surfaces These shock waves break the fouling. The of the mould without destroying the surfaces phenomenon is the same that is on propellers themselves. Studies demonstrate that then, in and pump impellers, which are destroyed if the case of austenitic steel, the high stresses made to work at excessive speed. caused by the ultrasound are absorbed by the Ultrasound ranging from 20 to 100 kHz is transformation of the surface layer from used in industry as it has the ability to cause austenite γ to ε-martensite (D’Oliveira, cavitation (Pilli et al., 2011; Rastogi 2011). 2003). Moreover, with an appropriate

540 C. Fragassa, M. Ippoliti surface finishing, it has an incubation period and chemical attack (Bignozzi et al., 2016). of erosion of more than 30 hours Typically used are acid agents and alkaline (D’Oliveira, 2003), well above the few agents, for the reasons previously treated minutes needed to cleaning. To ensure the (Figure 23). Ultrasonic cleaning is the only highest quality of cleaning, ultrasound is technology that is able to clean the spring- used in combination with high temperature vent.

Figure 23. Example of Injection Mould Ultrasonic Cleaning Machine with a multi stage treatment (Courtesy of Keymical)

4. Conclusions of inorganic elements in the compound made obsolete alkaline washing machine, and has The increased performance of the , made necessary the development of increasing demands of high performance and innovative cleaning system, which are the security and, especially in recent years, the dry ice and laser. The adoption of the spring- demand of a better external appearance fuel vents has led to the recovery of an old efficiency, have led to a constant evolution cleaning technology, that is the ultrasonic of the tires in terms of material, construction cleaning. It permits to obtain a very high and tread design. The majority-required level of quality finishing (Figure 24), quality has also led to an evolution in the together with several other useful benefits at field of tire moulds, in particular with the the level of process and products. adoption of the spring-vents. Ultimately, this A final comparison between all the detailed has caused the demand for more efficient technologies is reported in Table 5. mould cleaning systems. The extensive use

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Figure 24. Comparing tire surfaces before and after the Injection Mould Ultrasonic Cleaning (Courtesy of Keymical)

Table 5. Overview of tire mould cleaning technologies Spring- On Man Mould Clean Clean Final Technology vent board Power H&S erosion level Speed Score cleaning cleaning Needs Automatic YES High NO NO NO < 2H Good 4 sandblasting Hand YES High NO NO YES > 2H Good 2 sandblasting Automatic dry NO High NO YES NO < 2H Good 6 ice Hand dry ice NO High NO NO YES > 2H Good 3 Laser NO Medium NO YES NO < 2H Good 5.5 Alkaline NO Low NO NO NO > 2H Poor 2.1 washing Ultrasonic NO High YES NO NO < 2H Good 6

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Cristiano Fragassa Martin Ippoliti University of Bologna, University of Bologna, Department of Industrial Department of Industrial Engineering Engineering Forlì Forlì Italy Italy [email protected]

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